A typical wire electric discharge machining apparatus is equipped with an automatic wire threader. The automatic wire threader threads a wire electrode through an upper wire guide, a start hole, and a lower wire guide, in this order. The wire electrode threaded through the lower wire guide is delivered to a wind-up roller. The start hole has a diameter of approximately 1 mm, and is formed in the work before machining. The wire guide has a guide hole of 0.205 mm in diameter or 0.21 mm in diameter if the wire electrode to be used has a diameter of 0.20 mm. The upper and the lower wire guides are housed in their respective guide assemblies. Each guide assembly includes a flush nozzle for supplying work fluid to a work gap. The flush nozzle typically has an aperture of approximately 6 mm in diameter.
A current automatic wire threader includes a jet nozzle for generating a fluid jet (hereinafter referred to as a “jet”) coaxially with a wire electrode. The jet urges the wire electrode to move to a lower wire guide under constraint. A typical jet nozzle has an aperture of 1.5 mm in diameter. The jet nozzle is usually disposed in an upper guide assembly for housing an upper wire guide.
The automatic wire threader described in Japan laid open publication No. H07-029246 includes a guide pipe movable vertically. A wire electrode can be threaded through the guide pipe and high pressure fluid is supplied to the guide pipe. A typical guide pipe has an outer diameter of 2 mm and an inner diameter of 1 mm. The guide pipe can easily approach a start hole, thereby enhancing the success rate of automatic wire threading. In most cases, a separable upper wire guide is used to thread a guide pipe therethrough. It is believed that such a separable wire guide has low durability and poor abrasion resistance.
The Japan laid open publication No. H05-021693 discloses a whirling arm whose one end is provided with a jet nozzle hole. The jet nozzle hole is movable between a retracted position and a lower portion of an upper guide assembly. The jet nozzle hole is located at the retracted position apart from the upper guide assembly during machining. One end of the whirling arm is fit in a lower portion of the upper guide assembly for the purpose of automatic wire threading. At this time, the jet nozzle hole is located immediately below a flush nozzle. The jet nozzle hole is separable so as to return to the retracted position. The jet nozzle hole is opened or closed by a pneumatic cylinder. Since the jet nozzle hole is provided outside the upper guide assembly, there is no limitation on designing of the upper guide assembly. In addition, no separable wire guide is necessary.
The Japan laid open publication No. 2010-005719 discloses an automatic wire threader having a jet nozzle hole provided outside an upper guide assembly, as similar to that of Japan laid open publication No. H05-021693. The jet nozzle set forth in the Japan laid open publication No. 2010-005719 is movable at higher speed between a retracted position and a lower portion of an upper guide assembly. The jet nozzle is separable on a plane including a central axis of the jet nozzle hole. The jet nozzle hole is opened or closed by a cylinder unit. The jet nozzle can be firmly coupled to a flush nozzle in a fluid-tight manner. The jet nozzle has a cylindrical portion perpendicularly extending downward. The cylindrical portion has a smaller outer diameter than that of the flush nozzle, so that the jet nozzle can approach a work more closely than the flush nozzle does.